Canadian special forces have temporarily suspended military assistance to Iraqi troops due to tensions between the Middle Eastern country's military and Kurdish fighters, the defense ministry said Friday.

The offensive, which follows weeks of soaring tensions between two US allies in the battle against the Islamic State, aims to retake oil fields and military bases that Kurdish forces seized during the fightback against the jihadists.

US-backed fighters said Friday they were preparing for a final assault on the Islamic State group's Syrian bastion Raqa, likely next month, with new weapons and armoured vehicles promised by Washington.

The United States on Tuesday expressed "deep concern" over Turkish air strikes against Kurdish fighters in Syria and Iraq and said they were not authorized by the US-led coalition fighting Islamic State.

Turkish President Recep Tayyip Erdogan condemned the United States' support of Kurdish fighters in Syria after AFP pictures revealed US commandos wearing the insignia of a militia branded a terror group by Ankara.

Turkey will continue to strike back at Kurdish fighters of the Democratic Union Party (PYD) in Syria, Prime Minister Ahmet Davutoglu told German Chancellor Angela Merkel on Sunday, despite growing pressure on Ankara to stop the shelling.